1. Field of the Invention
The present invention relates to an electrophotographic printing apparatus such as a laser printer or copier.
2. Description of the Related Art
In the electrophographic printing apparatus provided with: a timing roller to convey the sheet sent from a sheet hopper at the predetermined timing; a register roller to convey the sheet in timed relationship with the timing at which the toner image formed on the photoreceptor drum surface reaches the transfer device; and a conveying apparatus to convey the sheet passed through the transfer device toward a fixing device, conventionally, the sheet conveying speed of the timing roller, register roller, and conveying apparatus is set to the same as a process speed.
By using FIG. 1, the schematic structure of the electrophotographic printing apparatus will be described below. The sheet accommodated in a sheet hopper 10, is sent out by a pick roller 9, and one sheet passed through double feeding prevention rollers 8a and 8b is sent into timing rollers 1a and 1b by conveying rollers 7a and 7b. The sheet sent out by the timing rollers 1a and 1b at the predetermined timing is sent into the stopped register rollers 2a and 2b.
Herein, in the sheet sent into the register rollers 2a and 2b, because, under the condition that the leading edge of the sheet comes into contact with the register rollers 2a and 2b, the trailing edge side is sent into the register rollers 2a and 2b by the timing rollers 1a and 1b, the deflection is formed between the timing rollers 1a and 1b and the register rollers 2a and 2b. By this deflection and the rigidity of the sheet itself, the posture of the sheet is corrected, and the leading edge of the sheet is corrected so that it comes straight into contact with the nip of the register roller 2a and the register roller 2b.
As described above, the sheet whose posture is corrected, is sent from the register rollers 2a and 2b in timed relationship with the timing at which the toner image (not shown) formed on the photoreceptor drum 3 surface reaches the transfer device 4.
The sheet holding the toner image which is transferred from the photoreceptor drum 3 and not yet fixed, is sent to the fixing device 6 through the conveying apparatus 5, and the toner image is thermally fixed onto the sheet by the fixing device 6.
When the fixed sheet is delivered outside the printing apparatus, the sheet is delivered through conveying rollers 20a, 20b, and conveying rollers 21a and 21b. Further, when the front and rear sides of the fixed sheet are reversed and the sheet is delivered outside the printing apparatus, the sheet sent from the conveying rollers 20a and 20b is introduced once into a sheet path side provided with conveying rollers 22a, 22b, 23a and 23b, then, the conveying rollers 22a, 22b, 23a and 23b are reversely rotated, and the sheet is sent to the conveying rollers 21a and 21b, and delivered. Further, when two-sided printing is conducted, the sheet sent from the conveying rollers 20a and 20b is introduced into the sheet path side provided with conveying rollers 22a, 22b, 23a and 23b, and then, the sheet is sent again to the timing rollers 1a and 1b through the two-sides conveying path (the path provided with conveying rollers 24a, 24b, 25a, 25b, 26a and 26b) branched from the sheet path, and thereby, two-sided printing is conducted.
However, in the above structure, when the sheet ream weight is large (the thickness of the sheet is large), or when the sheet length is large (A3 sheet), because, when the trailing edge of the sheet is separated from the timing roller and register roller, the deflection of the sheet is released, the vibration is generated on the sheet, and the vibration is transmitted along the sheet, and there is a disadvantage that the transfer blurring is generated when the toner image is transferred by the transfer device.